I'm fixing to paint my deck and I was wondering if there are any words of advice as far as exposure to sunlight while drying. It's hot today and sure to get up in the upper 90s. (I'm in North Texas). Oh. and I'm using semi-gloss, if that makes a difference.

Well, a lot depends on whether you're using a latex or oil based paint. Latex paints have a finicky film formation mechanism that can get screwed up by either excessively high or low temperatures. Oil based paints, on the other hand, have a much more robust film formation mechanism that's hard to screw up. You can paint outdoors with oil based paint in a Manitoba blizzard, and the paint will stay tacky all winter long, and then cure normally once the warm weather comes again in spring. You can't do that with latex paints. In fact, when I paint with oil based paints, I'll put the paint and brush or roller in my freezer while each coat dries. The very cold paint warms up quickly once it's spread on the surface, and cures normally.

If you're painting with a latex paint, there's more concern about the paint not sticking well or forming a proper film. Both excessively high day time temperatures (or high temperature of the surface being painted) as well as cold can screw up the film formation process in latex paints. It's generally recommended not to paint with latex paints below 10 deg. Celsius (50 deg. F) or above about 30 deg. C (86 deg. F), and painting in direct sunlight on a hot day adds to the problem because the surface being painted can be hotter than the outdoor air temperature.

If it's an oil based paint you're going to be painting with, then go for it. I'd be more concerned about you getting dehydrated than about what's going to happen to the paint. You might want to thin your paint a bit with mineral spirits to compensate for the more rapid evaporation of thinner from the paint.

Alas, it's latex. I called Behr support and they advised me to not paint when the ambient temp was over 90 F. It's a cool 70 something this morning so I knocked it out. Another tip I got was to use this product called Mold Armor Deck Wash. I just about killed myself a couple years ago following the instructions on the Behr paint can. Three times I applied various cleaning chemicals and got down and hand scrubbed every square inch of wood. I wish I knew then...

I should comment on freezing oil based paint because I've seen people simply cover oil based paint with plastic between coats, and that won't do much. They're thinking as long as they prevent the mineral spirits from evaporating, they'll prevent the oil based paint from drying. You can do that with latex paints, but oil based paints are a nuther kettle of fish.

Oil based paints will actually get heavier when first exposed to air because they will absorb O2 from the air, and the increase in weight is measurable. And, of course, it's that O2 from the air that reacts at locations where pairs of unsaturated sites are in close proximity that causes pairs of C-O-C crosslinks to form between those unsaturated sites. It's the formation of those C-O-C crosslinks that ultimately converts the liquid paint into a solid, thereby causing it to "dry".

When you're using a brush or roller to paint with oil based paint, by the time you've put on a first coat, you can pretty well assume that plenty of O2 from the air has already dissolved in the paint that's in your brush or roller. So, once that happens, then the curing of the paint is just a matter of time. But, you can greatly stretch out the time for that paint to "dry" be getting it cold. Just like any other chemical reaction, the colder the reactants, the less quickly they'll react. In this case, the colder the paint, the less rapidly it'll react with the O2 dissolved in it. So, by putting the paint in the freezer, you can very much slow down the amount the paint "drys" between successive coats.

If you're painting with latex paint, then you can just cover the paint to prevent water and coalescing solvent from evaporating from the paint. Getting the paint cold would probably also help by minimizing the amount of water and coalescing solvent that evaporates from the latex paint, but you want to use the fridge, not the freezer. Freezing latex paint MIGHT wreck it. But, with latex paints, preventing the evaporation of the water is really all you need to do to prevent the paint from drying, so plastic bags are all you really need.

And, you can use this info to your advantage. If you want to prevent a skin forming on your oil based paint in storage, then when you're ready to paint, then pour some paint out of the can into a tray or whatever, and then put something inside the can to displace the air, and then put the paint can lid back on quickly. The whole idea is to minimize the time of exposure of the paint in the can to air.

Lee Valley sells a pressurized bottle of gas (mostly nitrogen) to displace the air in the can, but I prefer to use butane because I can buy it quite cheap at smoke shops for refilling butane lighters. Also, I know butane is heavier than air, (but I think nitrogen is almost the same density as oxygen). So, in the empty space in the can, the butane will settle to the bottom of that space, effectively isolating any air on top from the paint. But, it's important to remember to keep the exposure time of the paint to air to a minimum. If you leave the paint can open for an hour or two while you paint, and then just spray some butane or nitrogen into the can before closing it, that's a waste of butane or nitrogen. The paint at the top will cure to form a skin despite the butane or nitrogen over top of it.

Also, if you ever accidentally knock over a gallon of paint onto a floor or whatever, immediately cover the spill with plastic (or anything else that will prevent evaporation from the latex paint and/or O2 absorbtion by oil based paint. You stand a much better chance of being able to clean up the spill by keeping either kind of paint from drying, and the very first step in the drying of oil based paints is that absorbtion of O2 molecules from the air.